1. Field of the Invention
This invention pertains to the field of footwear and in particular pertains to athletic footwear.
2. Description of the Related Art
When running, a person pushes off on the toe of their foot, arcs their foot through the air and sets their foot down on the ground in front of their body. For most runners, their heel strikes first, and their foot pronates slightly as they roll forward onto the ball of the foot; they then repeat the process by pushing off on the ball of their foot or toes. This heel-to-toe motion is common among long-distance runners. When the heel strikes the ground, significant impact forces are created that must be absorbed by the runner and his shoes, preferably the latter. Without proper absorption mechanisms built into the shoe, these impact forces can create acute injuries.
At the moment of ground contact, the foot is generally inverted approximately 6 degrees, dorsi-flexed 10 degrees, and outwardly rotated 12 degrees. A 150 pound runner regularly generates impact forces in the order of 450 pounds force (3 G's).
To lessen a runner's potential injury by reducing the impact upon the runner, a running shoe must attenuate the impact. Since the impact force is the overall force divided by time of force application, the most efficacious method of absorbing shock is by extending the time of force application, and thereby lessening the impact upon the runner. This can be done by allowing for travel in the heel as it strikes the ground. This curtails the amount of shock communicated to the runner's body .
Some prior art running shoes address the problem of shock absorption by using a variety of micro-cellular foams, gels or air bladders, which offer minimal travel. Softer soles provide more cushion and shock absorption, but in so doing compromise the angular stability of the foot. Conversely, firmer soles better stabilize the foot, but provide commensurately less shock absorption.